1. Technical Field
The technology described herein relates generally to wireless networking. More particularly, the technology relates to simultaneous communications between stations in a wireless network using one or more of Multi-User (MU) Multi-Input-Multi-Output (MIMO) and MU Orthogonal Frequency Division Multiple Access (OFDMA) technologies.
2. Description of the Related Art
Wireless LAN (WLAN) devices are currently being deployed in diverse environments. Some of these environments have large numbers of access points (APs) and non-AP stations in geographically limited areas. In addition, WLAN devices are increasingly required to support a variety of applications such as video, cloud access, and offloading. In particular, video traffic is expected to be the dominant type of traffic in many high efficiency WLAN deployments. With the real-time requirements of some of these applications, WLAN users demand improved performance in delivering their applications, including improved power consumption for battery-operated devices.
Devices in a WLAN may use rate adaptation to provide reliable communication and high data throughput. Rate Adaptation may select a Modulation and Coding Scheme for a transmission according to information about a channel being used to carry the communication.
A WLAN is being standardized by the IEEE (Institute of Electrical and Electronics Engineers) Part 11 under the name of “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.” A series of standards have been adopted as the WLAN evolved, including IEEE Std 802.11™-2012 (March 2012) (IEEE 802.11n). The IEEE Std 802.11 was subsequently amended by IEEE Std 802.11Ae™-2012, IEEE Std 802.11Aa™-2012, IEEE Std 802.11Ad™-2012, and IEEE Std 802.11Ac™-2013 (IEEE 802.11ac).
Recently, an amendment focused on providing a High Efficiency (HE) WLAN in high-density scenarios is being developed by the IEEE 802.11ax task group. The 802.11ax amendment focuses on improving metrics that reflect user experience, such as average per station throughput, the 5th percentile of per station throughput of a group of stations, and area throughput. Improvements may be made to support environments such as wireless corporate offices, outdoor hotspots, dense residential apartments, and stadiums.
Orthogonal Frequency Division Multiple Access (OFDMA) can be used in wireless networks to enhance the aggregation into a single frame of multiple payloads that are destined to or transmitted from multiple stations. Accordingly, OFDMA technology is now being considered for next generation WLAN technologies, including the 802.11 ax HE WLAN.
An HE WLAN supports Down-Link (DL) and Up-Link (UL) Multi-User (MU) transmissions such as MU OFDMA transmissions and MU Multi-Input-Multi-Output (MU MIMO) transmissions.
In an UL MU transmission, an Access Point (AP) may transmit a frame requiring an immediate response to a plurality of stations, such as a trigger frame or another type of frame. In response, the plurality of stations simultaneously transmit respective UL MU transmission frames, referred to herein as trigger-based frames, to the AP.
In an UL MU OFDMA transmission, the AP schedules which sub-bands (or a group of sub-carriers) each station uses to transmit their payload as part of the UL MU OFDMA transmission to the AP. The allocation of sub-carriers to stations can offer link gain if done judiciously. Particularly, when the AP knows the channel conditions for the group of sub-carriers with respect to each station, then higher throughput may be achieved by allocating a group of sub-carriers with better respective channel conditions to the respective stations, and by avoiding allocating sub-carriers with worse respective channel conditions to the respective stations, where “better” and “worse” are relative to other groups of sub-carriers. The suitability of the respective group of sub-carriers for use by a station is called frequency-selectivity status of the station, and the AP may advantageously acquire frequency-selectivity information for each station by an appropriate means.
One type of process for acquiring frequency-selectivity information exchanges between two wireless devices so that a recipient wireless device that receives a first frame can provide, using a second frame, a transmitting wireless device that transmitted the first frame with channel quality information for each of the subcarriers that the first frame spanned. Such a process is referred to as an explicit process.
Another type of process for acquiring frequency-selectivity information obtains the channel quality information without exchanging additional frames or in fact without inserting additional content to any frame. This may be performed by taking advantage of the reciprocity of the wireless channel when both nodes exchange frames in the same wireless channel, such as may be done in a WLAN operating according to an IEEE 802.11 standard. Such a process is referred to as an implicit process.
When one or more responding stations are scheduled to send respective trigger-based frames as responses to a trigger frame sent by an AP, rate adaptations for the trigger-based frames may be performed by the AP. However, the rate adaptations for the trigger-based frames performed by the AP may not produce as high a data rate or as high a reliability as rate adaptations for the trigger-based frames performed by the respective responding stations would. Furthermore, providing additional information to the AP at least helps to perform the rate adaptation more efficiently. On the other hand, and in particular for an UL MU OFDMA trigger-based frame where the AP is performing the rate adaptations for a large number of responding stations participating in the UL MU OFDMA trigger-based frame, a rate adaptation process may not be scalable. As a result, the AP may not be able to perform the correct rate adaptations for the large number of responding stations.